I found the same thing. Blacks tip OK, not great, very little if it is a group of black women. Perhaps they tip a black server better, I don’t know.
Any group of women don’t tip well, but they tip better if the server is male and especially if he is attractive and flatters them. Ditto the other way around with males customers and female servers. That’s what I observed.
Often when a group of women came in and were to be put in a female server’s section, she would “trade” that table to a male server. The male server knew he could make more and possible do very well with an all female table. Sometimes he would ‘tip’ the intended server a portion of his tip for giving him the table.
That’s just the way it works.
Relatives who have been servers tell me that immigrants are also lousy tippers.
And a rapidly growing part of the customer base.